The drivetrain in many light-duty and sport-utility vehicles includes a transfer case for transmitting drive torque to all four wheels of the vehicle, thereby establishing a four-wheel drive mode of operation. To accommodate differing road surfaces and conditions, some transfer cases are equipped with a gear reduction unit that allows the vehicle operator to selectively shift between four-wheel high-range and low-range drive modes. In many instances, however, the vehicle must be stopped before the transfer case can be shifted between the four-wheel high-range and low-range drive modes. For transfer cases that do not have a synchronized range shift mechanism, stopping the vehicle allows the relative velocity between the gears being moved into meshed engagement to be reduced to an acceptable level (i.e., synchronized) before initiating the range shift. Attempting to perform a range shift without first synchronizing the rotation of the gears may cause undesirable noise (“NVH”) as well as physical damage to the transfer case.
There may be instances, however, where stopping the vehicle to perform a range shift is inconvenient, particularly upon encountering road conditions and surface terrains where maintaining the vehicle's rolling momentum would assist in overcoming the adverse conditions encountered. To alleviate this problem, some gear reduction units are adapted to permit the vehicle operator to shift between four-wheel high-range and low-range drive modes without having to stop the vehicle. One means for accomplishing this is by incorporating a device commonly known as a synchronizer into the range shift mechanism. A synchronizer is a device that temporarily prevents rotating gears from entering into meshed engagement with one another until after the synchronizer adjusts the rotational velocities of the gears so as to be substantially equal. Once the rotational velocities are substantially equal, the synchronizing mechanism allows the gears to enter into meshed engagement, thereby completing the gear shift.
Generally it is desirable to complete a range shift operation in as short a time period as possible. The speed at which a range shift can be completed is dependant on various factors, including the maximum speed at which the range shift mechanism can be operated as well as the speed at which the range shift mechanism should be operated during the synchronization process. Operating the range shift mechanism too quickly during the synchronization process may not allow sufficient time for synchronization to occur, which may result in undesirable and potentially damaging gear clashing. On the other hand, performing a range shift at too low of a shift speed will result in unnecessarily long shift durations. Prior shift mechanism designs have attempted to resolve this dilemma by limiting the speed of the range shift to a maximum shift speed at which synchronization can occur without causing excessive NVH. But since the shift velocity required for synchronization is often less than the maximum shift velocity at which the range shift mechanism can be operated, the time required to complete a range shift is still greater than if the range shift mechanism were operated at its maximum shift velocity. Accordingly, the time required to complete a range shift can potentially be reduced by allowing a synchronized range shift mechanism to be operated at a shift velocity greater than the shift velocity required for synchronization. Thus, a recognized need exists for developing a method for operating a synchronized range shift mechanism that is capable of exploiting the range shift mechanism's maximum operating speed while meeting the operational requirements of the synchronization process.